1. Field of the Invention
The invention is directed to an electric hoist with a lifting speed that is motor-speed-regulated. More particularly, the invention relates to an electric chain hoist, in which an electric motor, a gear unit, a wheel around which hoisting means are looped, the hoisting means, and load-carrying means are connected with one another in a drive connection. An electric control unit is provided for the electric motor and is connected to a manual control switch.
2. Discussion of the Prior Art
Electric chain hoists of the above-mentioned type are used with hoisting mechanisms which are driven by an electric motor and use a link chain as carrying means, primarily for vertical transportation of loads.
The electrical energy is switched directly from the power supply for the motor by the manual control switch mentioned above or is controlled via contactors (motor switches). In the latter case, a suitable number of pairs of motor switches which are installed in a protected manner at the chain hoist are associated with the electric control in addition to the manual control switch. Electric (chain) hoists generally have a single speed for lifting and lowering the load corresponding to the nominal speed of the motor and the selected gear reduction. In special applications, a second, slower speed is required. In such cases, a pole-changeable motor is selected which has an additional winding for the second motor speed and is switched by additional switches of the electric control unit. The range of application of electric (chain) hoists and the convenience of operation could be substantially improved if the speed, i.e., the motor speed, could be changed continuously between very small values and the nominal speed. This is enabled in a known manner by means of a static frequency converter which transforms the fixed line or mains frequency determining the speed into a rotating field with variably controllable frequency for the motor. Such an arrangement is disclosed in EP 0529120 A1.
In this arrangement the frequency converter control replaces the electromechanical motor switches mentioned above. In so doing, the problem arises that the location for mounting the motor switch is not suitable for receiving a frequency converter or there is not sufficient space available in this location. In particular, losses must be guided off as heat and, further, it is necessary to provide means for measuring speed with appropriate accuracy.